In some areas, existing techniques for purifying waste water for reuse as potable water are limited by government regulations designed to ensure that harmful contaminants are not passed into the potable water supply. Such regulations can require at least a 5-log reduction in contaminants between the wastewater and the potable water. In the United States, some states can require additional barriers between waste and potable waters for certain dissolved contaminants, pathogens and emerging contaminants. For example, in some areas such as California, treated water must be injected into a groundwater basin for years before it can be pumped to the surface and used for potable drinking water. This approach is called Indirect Potable Reuse (IPR) and has also been used by other U.S. states, Australia, and other regulatory bodies, and is the state of the art for wastewater to potable water reuse.
In reverse osmosis (RO) systems, feed and product water conductivity is monitored in real-time using conductivity measurements to confirm that the membranes are removing a certain percentage of salts and this salt removal is used as a surrogate for overall membrane rejection and integrity. Even though this is real-time data, it is at a low resolution compared to what is needed for detecting acceptable levels of contamination or leaks (e.g., pinhole leaks) that may not be detected with currently used techniques for days, months, or years. Typically, if an RO system can remove greater than 99.0% of total salts (e.g., total dissolved solids), then a RO system is permitted for 2-log (99.0%) removal of pathogens and viruses. Consequently, one or more RO membrane elements in a RO system containing any number of RO membrane elements can have integrity issues that allow virus and pathogens to pass through the RO membrane via a small leak that is not detected as long as the overall system or subsystem achieves the overall salt rejection target. Pinhole leaks, O-ring leaks and other minor leaks may not be easily detected because the detection limit of conductivity. RO integrity issues may occur over the lifetime of a RO membrane (e.g., several months to 10 years) and never be identified as long as the overall system meets performance targets. Similar issues can occur with standard forward osmosis (FO) membranes.
Other existing membranes and membrane systems used to purify water, including: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), RO, and membrane distillation (MD) lack real-time, fail-safe methods for membrane integrity monitoring. This means that often there can be leaks in membrane filters that allow pathogens, virus, chemicals, and other unwanted contaminants to pass through these leaks for hours, days, or years before the integrity issue is detected.
In FO systems having pressurized feed streams, leaking from the feed stream into the draw stream may occur when membrane integrity is lost. Membrane integrity monitoring (e.g., for membrane leaks) and ultra-high purification are often inadequate in current membrane water purification systems. Membrane integrity monitoring generally refers to monitoring for and detecting leaks, defects in membranes, and/or other potential integrity issues that can cause pathogens and/or other undesired contaminants to go across the membrane when they would likely be rejected by the membrane if no integrity issue were present. There are problems with existing methods of FO treatment that inhibit membrane integrity monitoring and ultra-high purification. Existing methods for FO dewatering, filtration, and treatment use a pressurized feed approach such that the feed stream into and out of an FO unit is pressurized to a higher average hydrostatic pressure than the draw stream into and out of an FO unit which may allow unwanted contaminants to pass from the feed into the draw in the event of loss of membrane integrity. There are also limitations to membrane integrity monitoring in general within the water treatment industry.
Commercial MF and UF systems and similar membrane bioreactor (MBR) systems utilize air decay tests to monitor membrane integrity. This requires that the membranes be taken offline so that pressurized air can be pumped into the membrane housing and all inlets and outlets are closed to retain air pressure in the housing. The United States Environmental Protection Agency (USEPA) and other regulatory agencies have guidelines for a maximum amount of air pressure that can be released from the system in a certain amount of time for the membrane to be considered acceptable. If the membrane releases too much air pressure and fails the test, then it likely has a leak and is often replaced with a new membrane. However, because this is not a real time test, the membrane filter may have an integrity breach or leak for hours, days, or weeks before the integrity issue is identified.